The present invention is an improved hydrophone that operates on the principles of fiber optic interferometers, that is, interferometers in which the sensing function takes place within optic fibers that compose the legs of the interferometer. This is done as an external variable, here, the acoustic field, changes the optic length of the sensing leg while the reference leg is left unchanged. Typically, these interferometers utilize two or more light splitting and light adding couplers within the system. Normally an evanescent wave coupler splits the source light between the two legs and a second evanescent wave coupler recombines the two beams. The light intensity at the detector is the result of the constructive or destructive interference between those two beams of light, which depends on the relative optic lengths of the two legs, hence the strain in the sensing leg.
This invention relates to acoustical or mechanical configurations of fiber optic hydrophones for use with light sources and detection devices necessary for the interferometer to function as a useful sound-measuring device.
The earliest fiber optic interferometric hydrophones consisted of two coils of fibers, one exposed to the acoustic field, the other shielded from it. Because of the inherent insensitivity of the fiber to acoustic variations in this hydrostatic environment, the coils required fiber lengths of hundreds of meters to be able to detect signals near "sea state zero", the experimentally determined lowest ocean noise level. To keep the overall dimensions of the hydrophone within reasonable bounds, the fibers were wound into coils. However, optic fibers have increased losses if the radius of the bends are too sharp, and this usually puts a lower limit on coil diameters of one inch or larger. Also, the optic path length in the fibers is a strong function of temperature, and small temperature changes cause large noise signals in very long fibers.
The present invention provides a structure having a bending beam, which lessens both of these disadvantages. The bending beam can be configured as an acoustic diaphragm. For hydrophone use the lateral dimensions of the active parts can be reduced to less than one-fourth of an inch, and the length to two inches or less. In a line hydrophone for towed array purposes, this could reduce the overall diameter of the array's protective sheath to an inch or less, greatly reducing its weight and necessary storage space.
The present invention extends the advantages of a hydrophone constructed using the bending beam structure by increasing the sensitivity, if necessary, or increasing the ruggedness if sensitivity is adequate, and by reducing sensitivity to lateral noise accelerations because of its balanced design. A third advantage occurs from being able to adjust overall length dimensions of the hydrophone for purposes of integrating or averaging the acoustic signal, and for determining to some extent the frequency response limits of the device.